One-Pot Synthesis of Mikto Three-Arm AB2 Stars

Jul 17, 2012 - One-Pot Synthesis of Mikto Three-Arm AB2 Stars Constructed from. Linear and Macrocyclic Polymer Chains. Jakov Kulis, Zhongfan Jia, and ...
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One-Pot Synthesis of Mikto Three-Arm AB2 Stars Constructed from Linear and Macrocyclic Polymer Chains. Jakov Kulis, Zhongfan Jia, and Michael J. Monteiro* Australian Institute for Bioengineering and Nanotechnology, University of Queensland, Brisbane QLD 4072, Australia S Supporting Information *

ABSTRACT: Complex amphiphilic polymer architectures have very different self-assembly properties to their linear counterparts. Miktoarm stars have been the most studied in this regard but their synthesis is usually complex with many synthetic steps. Moreover, there are few examples of building 3-arms stars with cyclic building blocks. In this work, we have demonstrated the rapid (in 30 min) and highly efficient (at close to 99%) one-pot synthesis of mikto 3-arm AB2 star at 25 °C using two copper catalyzed “click”-type reactions (i.e., CuAAC and nitroxide radical coupling, NRC). By modulating the copper activity, the rates of NRC and CuAAC could be significantly changed. Under conditions where both the NRC and CuAAC reactions were similar, well-defined linear and cyclic building blocks consisting of linear-polystyrene, cyclic-polystyrene, poly(tert-butyl acrylate) and PEG could be combined to form a wide variety of chemically different AB2 stars. The log-normal distribution (LND) method was used to simulate the molecular weight distribution using experimental Mn, PDI and change in hydrodynamic volumes, and this method provided an additional and sensitive characterization method to the coupling efficiency for 3-arm formation. It allowed the determination of the type and amount of starting, two-arm or other high molecular weight species present after coupling. There were no adjustable parameters used in the LND simulations, making this a powerful method to characterize not only cyclic polymers but more complex architectures like 3-arm stars with very different chemical compositions. The LND simulations gave excellent agreement with the experimental MWDs, allowing us to determine the coupling efficiency of greater than 99% in most cases.

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INTRODUCTION Complex amphiphilic polymer architectures, including miktoarm star copolymers and dendrimers, self-assemble into micelles in water with aggregation behavior very different to linear diblock copolymers.1−5 Miktoarm star copolymers represent a structure in which polymer blocks (A and B in a binary system) are attached to a central core. In the simplest binary mikto 3-arm AB2 stars, the self-assembly can be very different to that of a linear diblock copolymer with the same volume fractions of blocks A and B.6,7 A versatile route to synthesize such architectures is by first preparing linear polymer chains with controlled chain length and end-groups by “living” radical polymerization.8−11 The end-groups are then converted to a chemical functionality that undergoes a facile and specific (in many cases a “click”) reaction to construct the architecture of choice. The most common and well established approach uses atom transfer radical polymerization (ATRP)9 to prepare well-defined polymer chains that are coupled together via the copper(I)-catalyzed azide−alkyne cycloaddition (CuAAC)12 reaction.1,3,13,14 This approach has produced multiblock copolymers,15 macrocycles,16−18 stars,1,19 dendrimers,2,3 bioconjugates and functional surfaces.20,21 Other cross-coupling reactions include strain-promoted azide−alkyne coupling (SPAAC),22 Diels−Alder,23 thio−bromo,24,25 and thiol− ene26−28 reactions. These robust orthogonal chemical reactions provide a method for selectively modifying macromolecules in © 2012 American Chemical Society

near quantitative yields to create complex architectures in a controlled and predictable way. Macrocyclic polymers represent unique structures that have the potential for use in the formation of high temperature stable micelles, mimicking the properties of cyclic lipids in the cell membranes of organisms that live in adverse environments (e.g., hot springs).29 Polymers (made by ATRP) cyclized through the CuAAC reaction are currently most widely used due to the simplicity of producing the linear polymer precursor with azide and alkyne end-groups.16,18 The resultant cyclic polymer can be coupled with itself or other linear polymers to form stars.17,30 However, this process requires many steps, including multiple chemical functional transformations and purifications to remove residual functional small molecules, which can interfere with the construction of miktoarm stars. Therefore, new and more attractive strategies for constructing miktoarm stars are required. Sugai et al31,32 have produced elegant cyclic structures from the chain-closure method. One reaction that has many attributes of a “click” reaction is the nitroxide radical coupling (NRC) reaction,33−35 which exclusively traps carbon-centered radicals at close to diffusion controlled rates. Polymer chains with halide end-groups (e.g., made by ATRP) can be coupled to nitroxides36−39 or Received: June 24, 2012 Published: July 17, 2012 5956

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Scheme 1. (A) General Synthetic Strategy for the Construction of AB2 3-Arm Miktoarm Stars (B) Structures of the AB2 Miktoarm Stars Synthesized (10−17)

reaction first; and second, DMSO was added to the reaction mixture to facilitate the NRC reaction. The ABC product was formed in high yields with very high coupling efficiency. In this work, we report on the one-pot synthesis of AB2 miktoarm stars built from linear and cyclic polymers using the CuAAC/NRC methodology at 25 °C (Scheme 1). We did not have to use the sequential CuAAC/NRC method as shown in our previous work43 since at least one reagent was a linear polymer, significantly reducing steric congestion around the core molecule. The range of polymers used included polystyrene (PSTY, both linear and cyclic), poly(tert-butyl acrylate) (PtBA) and poly(ethylene glycol) (PEG). Further to the synthetic work, we developed an accurate method to determine the purity of the 3-arm stars by fitting the experimental molecular weight distribution (MWD) with a simulated MWD based on a log-normal distribution (LND) model using a Gaussian function. Our methodology represents a versatile and highly practical strategy for the construction of not only miktoarm stars but also a wide range of more complex macromolecular architectures under mild conditions.

nitrones40 by using a copper catalyst to abstract the halide endgroup and form the carbon-centered radical. This coupling strategy in combination with CuAAC, which has been elaborated in our laboratory, has provided an easy way to construct dendrimers made from either small molecules41 or polymer42 building blocks. Alkoxyamines can be reversibly cleaved at elevated temperatures,36,38 and as such most products from the NRC reaction can be degraded if required for a desired application. A useful feature of the NRC/CuAAC methodology is that since both reactions are catalyzed by copper (e.g., CuI), their respective rates can be modulated through a judicious choice of ligand and solvent.41 The CuI activity for both reactions can be controlled and significantly changed, and this modulation of activity was utilized in the synthesis of dendrimers in one pot either divergently, convergently or in parallel without the need for multiple protection/deprotection steps. This represents a significant advance in dendrimer synthesis. Recently, we demonstrated the construction of a 3-miktoarm star (ABC) at 25 °C from three different cyclic polymers (i.e., c-polystrene, c-poly(tert-butyl acrylate) and c-poly(methyl acrylate)) using the NRC/CuAAC methodology in a multistep process.43 Initial attempts to produce the ABC structure using a one pot reaction (i.e., CuIBr/PMDETA in toluene or CuIBr/Me6tren in DMSO) gave incomplete formation most probably due to the steric hindrance of joining three cyclic polymers to a small core molecule. To produce the ABC 3-miktoarm star, a sequential method was used to kinetically overcome the steric hindrance: first, the reaction between the cyclic polymers and the core was catalyzed by CuIBr/PMDETA in toluene to give the CuAAC

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EXPERIMENTAL SECTION

Analytical Methods. Size Exclusion Chromatography (SEC) Calibrated with Linear PSTY. All polymer samples were dried prior to analysis under vacuum for 24 h at 25 °C. The dried polymer was dissolved in tetrahydrofuran (THF) to a concentration of 1 mg/mL and then filtered through a 0.45 μm PTFE syringe filter. Analysis of the molecular weight distributions of the polymers was accomplished using a Waters 2695 separations module, fitted with a Waters 410 refractive index detector maintained at 35 °C, a Waters 996 photodiode array detector, and two Ultrastyragel linear columns (7.8 5957

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× 300 mm) arranged in series. These columns were maintained at 40 °C for all analyses and are capable of separating polymers in the molecular weight range of 500−4 000 000 g/mol with high resolution. All samples were eluted at a flow rate of 1.0 mL/min. Calibration was performed using narrow molecular weight PSTY standards (PDI ≤ 1.1) ranging from 500 to 2 000 000 g/mol. Data acquisition was performed using Empower software, and molecular weights were calculated relative to polystyrene standards. Absolute Molecular Weight Determination by Triple DetectionSEC. Absolute molecular weights of polymers were determined using a Polymer Laboratories GPC50 Plus system equipped with dual angle laser light scattering detector, viscometer and differential refractive index detector. HPLC grade tetrahydrofuran was used as eluent at a flow rate 1 mL/min. Separations were achieved using two PLGel Mixed C (7.8 × 300 mm) SEC columns connected in a series and held at a constant temperature of 40 °C. The triple detection system was calibrated using a 2 mg/mL PSTY Standard (Polymer Laboratories: Mwt = 110K, dn/dc = 0.185 and IV = 0.4872 mL/g). Polymer samples of known concentrations were freshly prepared in THF and passed through a 0.45 μm PTFE syringe filter just prior to injection. The absolute molecular weights and dn/dc values were determined using Polymer Laboratories Multi-Cirrus software. The dn/dc values determined by the quantitative mass recovery technique using the Cirrus software were in good agreement with the theoretical value. Preparative Size Exclusion Chromatography (Prep-SEC). Linear PSTY was purified using a Varian ProStar preparative SEC system equipped with a manual injector, a differential refractive index detector and a single wavelength ultraviolet−visible detector. HPLC grade tetrahydrofuran was used as eluent at a flow rate of 10 mL/min. Separations were achieved using a PLgel 10 μm 103 Å, 300 × 25 mm preparative SEC column held at 25 °C. The dried impure polymer was dissolved in THF to give concentration of 100 mg/mL. This solution was filtered through a 0.45 μm PTFE syringe filter prior to injection. Fractions were collected manually and the composition of each was determined using the Polymer Laboratories GPC50 Plus system equipped with triple detection as described above. 1 H Nuclear Magnetic Resonance (NMR). All NMR spectra were recorded on a Bruker DRX 500 MHz spectrometer at 25 °C using an external lock (CDCl3) and referenced to the residual nondeuterated solvent (CHCl3). A DOSY experiment was run to acquire spectra to suppress any small molecule or solvent signals by increasing the pulse gradient and p30 from 1 to 2 ms. Attenuated Total Reflectance Fourier Transform Spectroscopy (ATR−FTIR). ATR-FTIR spectra were obtained using a diamond/ZnSe crystal ATR accessory on a Perkin-Elmer 400 FT-IR/FT-FIR Spectrometer. Spectra were recorded between 4000 and 500 cm−1 by acquiring 24 scans at 4 cm−1 resolution with an OPD velocity of 0.2 cm s−1. Solids were pressed directly onto the diamond internal reflection element of the ATR without further sample preparation. Matrix Assisted Laser Desorption Ionization−Time of Flight (MALDI−ToF) Mass Spectrometry. MALDI-ToF mass spectra were obtained using a Bruker Autoflex III Smartbeam TOF/TOF 200 system. All spectra were recorded in reflectron mode. For PSTY polymers trans-2-(3-(4-tert-butylphenyl)-2-methylpropenylidene)malononitrile (DCTB; 20 mg mL−1 in THF) was used as the matrix and Ag(CF3COO) (1 mg mL−1 in THF) as the cation source. For PtBA polymers trans-2-(3-(4-tert-butylphenyl)-2methylpropenylidene)malononitrile (DCTB; 20 mg mL−1 in THF) was used as the matrix and Na(CF3COO) (1 mg mL−1 in THF) as the cation source. For PEG polymers 2-(4′-hydroxybenzeneazo)benzoic acid (HABA) was used as the matrix and Na(CF3COO) (1 mg mL−1 in THF) as the cation source. All samples were prepared by cospotting the matrix (20 μL), salt (2 μL), and polymer (20 μL, 1 mg mL−1 in THF) solutions on the target plate. Low Resolution-Electrospray Ionization-Mass Spectrometry (LR− ESI−MS). All mass spectra were recorded on a Bruker Esquire HCT (High Capacity 3D ion trap) instrument with a Bruker ESI source and positive ion model.

Materials. The following monomers were deinhibited before use by passing through a basic alumina column: styrene (STY: Aldrich, >99%) and tert-butyl acrylate (tBA: Aldrich, >99%). The following reagents were used as received: alumina, activated basic (Aldrich: Brockmann I, standard grade, ∼ 150 mesh, 58 Å), dimethyl(amino)pyridine (DMAP, Aldrich, 99%), magnesium sulfate, anhydrous (MgSO4: Scharlau, extra pure), potassium carbonate (K2CO3: AnalaR, 99.9%), silica gel 60 (230−400 mesh ATM (SDS)), sodium hydrogen carbonate (Merck, AR grade), triethylamine (TEA: Fluka, 98%), 2-bromopropionyl bromide (BPB: Aldrich, 98%), 2-bromoisobutyryl bromide (BIB, Aldrich, 98%), propargyl bromide solution (80 wt % in xylene, Aldrich), propargyl ether (Aldrich, 99%), p-toluenesulfonyl chloride (Aldrich, ≥ 98%), sodium azide (NaN3: Aldrich, ≥ 99.5%), TLC plates (silica gel 60 F254), 1,1,1(trihydroxymethyl) ethane (Aldrich, 96%), Sodium hydride (60% dispersion in mineral oil) The following solvents were used as received: acetone (ChemSupply, AR), chloroform (CHCl3: Univar, AR grade), dichloromethane (DCM: Labscan, AR grade), diethyl ether (Univar, AR grade), dimethyl sulfoxide (DMSO: Labscan, AR grade), N,Ndimethylformamide (DMF: Labscan, AR grade), ethanol (EtOH: ChemSupply, AR), ethyl acetate (EtOAc: Univar, AR grade), hexane (Wacol, technical grade, distilled), hydrochloric acid (HCl, Univar, 32%), anhydrous methanol (MeOH: Lichrosolv, 99.9%, HPLC grade), Milli-Q water (Biolab, 18.2 MΩm),, tetrahydrofuran (THF: Labscan, HPLC grade), toluene (HPLC, Labscan, 99.8%). The following polymers, initiators, ligands, and metals were used as received for the various polymerizations unless otherwise stated: poly(ethylene glycol) monomethyl ether (PEG−OH), Aldrich, Mn = 2000), methyl-2-bromopropionate (MBP, Aldrich, 98%), ethyl-2bromoisobutyrate (EBiB, Aldrich, 98%), N,N,N′,N′,N″-pentamethyldiethylenetri-amine (PMDETA: Aldrich, 99%), copper(I) bromide (Cu(I)Br: MV Laboratories, INC., 99.999%), copper(II) bromide (CuBr2: Aldrich, 99%). Synthesis of Alkyne(hydroxyl) ATRP Initiator. The alkyne(hydroxyl) ATRP initiator was synthesized according to the literature procedure.44

Synthesis of Trifunctional Core (1). The trifunctional core (1) was synthesized according to the literature procedure previously reported by our group.42

Synthesis of CuIIBr2/PMDETA Complex. Copper(II) bromide (4.10 g, 1.84 × 10−2 mol) was stirred in MeOH (200 mL) until complete dissolution was achieved. To this stirred solution PMDETA (3.20 g, 1.85 × 10−2) was added dropwise after which the solution was stirred for 1 h at room temperature. The reaction mixture was concentrated to ∼30 mL and diethyl ether (20 mL) was added to the solution until the complex just started to precipitate. The mixture was kept in the refrigerator overnight, more diethyl ether added (∼10 mL) and the precipitated complex collected by vacuum filtration, washed with diethyl ether and dried in vacuo for 24 h at 25 °C. Synthesis of Polymers. Synthesis of PSTY24−Br (2). Styrene (15.39 g, 0.1480 mol), PMDETA (0.351 mL, 1.68 × 10−3 mol), methyl-2-bromopropionate (0.374 mL, 3.36 × 10−3 mol) and CuIIBr2/ 5958

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stirred for 3 h. After stirring the reaction contents were exposed to air and solvent evaporated to dryness using air flow. The residue was then dissolved in chloroform and extracted with H2O (×3) to remove excess copper salts. The organic layer was then dried with Na2SO4, reduced in volume under vacuum and passed through activated basic alumina. The polymer was then recovered by precipitation into 10× volume of MeOH. The resulting white precipitate was collected by vacuum filtration and dried under vacuum. Purity by SEC = 93%. The crude product was fractionated by preparative SEC and fractions combined and characterized by linear PSTY calibrated SEC (Mn = 2230, PDI = 1.08) and triple detection SEC (Mn = 3190, PDI = 1.07). Synthesis of c-PSTY28−Br (4). c-PSTY28−OH (Mn = 3190, PDI = 1.07, 0.540 g, 1.69 × 10−4 mol), TEA (0.382 mL, 2.755 × 10−3 mol) and dry THF (26 mL) were added to an argon purged Schlenk flask. The contents were cooled in an ice bath under Ar. To this stirred mixture, a solution of bromopropionyl bromide (BPB) (0.289 mL, 2.76 × 10−3 mol) in dry THF (4 mL) was added dropwise under argon via an airtight syringe over 15 min. After addition the ice-bath was removed and the contents were stirred at room temperature for 56 h. After stirring, the reaction mixture was filtered through filter paper and concentrated under reduced pressure. The polymer solution was precipitated into 10× volume of cold 70/30 MeOH/H2O. The polymer was recovered by vacuum filtration to yield a white solid. Linear PSTY calibrated SEC (Mn = 2420, PDI = 1.08) and triple detection SEC (Mn = 3180, PDI = 1.05). Synthesis of c-PSTY28−N3 (5). NaN3 (33.2 mg, 5.10 × 10−4 mol) was added to a stirring solution of c-PSTY28−Br (4) (Mn = 3180, PDI = 1.05, 150 mg, 4.72 × 10−5 mol) in DMF (8.0 mL). The reaction mixture was stirred for 20 h at 25 °C. The reaction volume was reduced by half by placing under a stream of air and the polymer was precipitated in 10× volume of 80/20 MeOH/H2O. The polymer was recovered by vacuum filtration and washed exhaustively with water and MeOH. The polymer was redissolved in DCM, reprecipitated in 10× volume of MeOH and recovered by vacuum filtration. The white solid was dried under vacuum. SEC (Mn = 2420, PDI = 1.09), triple detection SEC (Mn = 3110, PDI = 1.11). Synthesis of Polymers. Synthesis of PtBA25−Br (6). tBA (10.0 g, 7.80 × 10−2 mol), PMDETA (0.627 mL, 3.00 × 10−3 mol), ethyl-2-

PMDETA complex (0.133 g, 3.36 × 10−4 mol) were added to a Schlenk flask and purged with argon for 30 min with vigorous stirring. CuIBr (0.241 g, 1.68 × 10−3 mol) was added under a positive argon flow and the contents bubbled with argon for 5 more min. The reaction vessel was then sealed, placed in an oil bath at 80 °C and the reaction mixture stirred for 4.5 h. The reaction was terminated by quenching in ice followed by exposure to air. The contents were diluted with dichloromethane and passed through activated basic alumina. The solvent was removed under reduced pressure and the residue dissolved in a minimal amount of dichloromethane. The polymer was precipitated in 10× volume of MeOH. The resulting white precipitate was collected by vacuum filtration and dried under vacuum (Mn = 2690, PDI = 1.11). Synthesis of PSTY24−N3 (3). NaN3 (0.772 g, 1.19 × 10−3 mol) was added to a stirring solution of PSTY24−Br (2) (Mn = 2690, PDI =

1.11, 0.320 g, 1.19 × 10−4 mol) in DMF (2.0 mL). The reaction mixture was stirred for 20 h at 25 °C. The polymer was precipitated in 10x volume of 10% water/MeOH, recovered by vacuum filtration and washed exhaustively with water and MeOH. The polymer was redissolved in DCM, reprecipitated in 10x volume MeOH and recovered by vacuum filtration. The polymer was dried under vacuum. Triple detection SEC (Mn = 2670, PDI = 1.11). Synthesis of c-PTSY. Synthesis of l-(HO)−PSTY28−Br. Styrene (12.0 g, 0.115 mol), PMDETA (0.251 mL, 1.20 × 10−3 mol), CuIIBr2/ PMDETA (0.0952 g, 2.40 × 10−4 mol) and alkyne(hydroxyl) initiator, 2-((2-hydroxyethyl)(prop-2-yn-1-yl)amino)ethyl 2-bromo-2-methylpropanoate, (0.701 g, 2.40 × 10−3 mol) were added to a Schlenk flask equipped with a magnetic stirrer and purged with argon for 30 min with vigorous stirring. CuIBr (0.172 g, 1.20 × 10−3 mol) was added under a positive argon flow and the contents purged with argon for a further 5 min. The reaction vessel was then sealed, placed in an oil bath at 80 °C and the reaction mixture stirred for 4.5 h. The reaction was terminated by quenching in ice followed by exposure to air and dilution with dichloromethane (ca. 3-fold to the reaction mixture volume). The copper salts were removed by passage through activated basic alumina. The solvent was removed under reduced pressure and the residue dissolved in a minimal amount of dichloromethane. The polymer was precipitated in 10× volume of MeOH. The resulting white precipitate was collected by vacuum filtration and dried under vacuum. Triple detection SEC (Mn = 2950, PDI = 1.08). Synthesis of l-(HO)−PSTY28−N3. NaN3 (0.90 g, 1.38 × 10−2 mol) was added to a stirring solution of l-(HO)-PSTY28−Br (Mn = 2950, PDI = 1.08, 4.00 g, 1.36 × 10−3 mol) in DMF (40 mL). The reaction mixture was stirred for 20 h at 25 °C. The polymer was precipitated in 10× volume of 10% water/MeOH, recovered by vacuum filtration and washed exhaustively with water and MeOH. The polymer was redissolved in DCM, reprecipitated in 10× volume of MeOH and recovered by vacuum filtration. The polymer was dried under vacuum. Triple detection SEC (Mn = 3140, PDI = 1.09). Synthesis of c-PSTY28−OH. A solution of l-(HO)−PSTY28−N3 (Mn = 3140, PDI = 1.09, 0.40 g, 1.27 × 10−4 mol) in toluene (20 mL) was purged with argon for 30 min to remove oxygen. The polymer solution was added via syringe pump, at a flow rate of 2.5 mL/min, to a deoxygenated solution of CuIBr (0.976 g, 6.80 × 10−3 mol) and PMDETA (1.42 mL, 6.80 × 10−3 mol) in toluene (20 mL) at 25 °C. After the addition of the polymer solution the reaction mixture was

bromoisobutyrate (EBiB) (0.445 mL, 3.00 × 10−3 mol) and CuIIBr2/ PMDETA complex (0.238 g, 6.00 × 10−4 mol) were added to a Schlenk flask and purged with argon for 30 min with vigorous stirring. CuIBr (0.241 g, 1.68 × 10−3 mol) was added under a positive argon flow and the contents purged with argon for a further 5 min. The reaction vessel was then sealed, placed in an oil bath at 50 °C and the reaction mixture stirred for 80 min. The reaction was terminated by quenching in ice followed by exposure to air and dilution with dichloromethane. The copper salts were removed by passage through activated basic alumina. The solvent was removed under reduced pressure and the residue dissolved in a minimal amount of dichloromethane. The polymer was precipitated in 10x volume of cold 50/50 MeOH/H2O and placed in the fridge for 3 h. The matrix solution was decanted and the solid residue was dissolved in dichloromethane, dried with Na2SO4 and filtered. The solvent was removed under reduced pressure and the resulting clear residue was dried under vacuum. Linear PSTY calibrated SEC (Mn = 2800, PDI = 1.11) and triple detection SEC (Mn = 3400, PDI = 1.11). Synthesis of PtBA25−N3 (7). NaN3 (0.169 g, 2.60 × 10−3 mol) was added to a stirring solution of PtBA25−Br (6) (Mn = 3410, PDI = 1.12, 0.909 g, 2.67 × 10−4 mol) in DMF (7.0 mL). The reaction mixture was stirred for 20 h at 25 °C. The polymer was precipitated into 10x volume of cold 50/50 MeOH/H2O and placed in the fridge for 3 h. 5959

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polymer−N3 (PSTY24−N3 (3) or PtBA25−N3 (7) or PEG47−N3 (9) or c-PSTY28−N3 (5)) (1.3 equiv) and PMDETA (3 equiv) were placed in a Schlenk flask and dissolved in 50/50 v/v DSMO/toluene (∼100 mg of polymer/mL). Oxygen was removed from the solution by purging with argon (30 min). CuIBr (3 equiv) was added under a positive argon flow and the reaction vessel was sealed and placed in an oil bath at 25 °C with stirring for up to 3 h. The contents were diluted with chloroform and passed through activated basic alumina. The solvent was removed under reduced pressure to yield a residue. The crude product was fractionated by preparative SEC and fractions combined and precipitated into a suitable solvent and characterized by SEC. After preparative SEC, product 10 was precipitated in cold mixture of MeOH and H2O (80/20 v/v), products 11, 13, 15, and 17 were precipitated in hexane, and products 12, 14, and 16 were precipitated in cold mixture of MeOH and H2O (50/50 v/v). For the synthesis of the miktoarms (12) P(STY24-b-(tBA25)2) and (16) P(cSTY27-b-(tBA25)2), a reaction solvent mixture of 60/40 v/v toluene/DMSO was used due to the partial solubility of the larger amount of PtBA25-Br in the 50/50 toluene/DMSO solvent mixture. Miktoarm stars 10−17 were obtained with a recovered yield of approximately 40%. Yields were as follows: structure 10= 42%, 11 = 38%, 12 = 40%, 13 = 39%, 14 = 37%, 15 = 42%, 16 = 41%, and 17 = 42%.

The matrix solution was decanted and the solid residue was dissolved in dichloromethane, dried with Na2SO4 and filtered. The solvent was removed under reduced pressure and the resulting clear residue was dried under vacuum. Triple detection SEC (Mn = 3430, PDI = 1.12). Chain-End Modification of PEG−OH. Synthesis of PEG47−Br (8). Poly(ethylene glycol) monomethyl ether (PEG47−OH) (Mn =

2000, 5.0 g, 2.5 × 10−3 mol) was dissolved in 250 mL of dry toluene and stirred until completely dissolved with minimal heating up to 40 °C. After contents cooled to room temperature, triethylamine (697 μL, 5.0 × 10−3 mol) was added and the reaction mixture stirred for 15 min at room temperature. Bromopropionyl bromide (BPB) (1.079 g, 5.0 × 10−3 mol) was then added dropwise at room temperature. A precipitate started to form on addition and a slight yellow/orange colored solution resulted. The reaction mixture was stirred for 48 h at room temperature. After stirring the contents were gravity filtered using filter paper and reduced in volume to a quarter (∼70 mL) under reduced pressure. The contents were then precipitated into 10x volume of diethyl ether. A white, waxy substance was collected by vacuum filtration. Triple detection SEC (Mn = 2240, PDI = 1.06). Synthesis of PEG47−N3 (9). Tosylation of PEG47−OH. Poly(ethylene glycol) monomethyl ether (PEG47−OH) (Mn = 2000,

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RESULTS AND DISCUSSION Synthesis of Polymer Precursors. The range and diversity of the polymer building blocks used in this work is given in Scheme 1. ATRP was first used to produce PSTY24−Br (2) using methyl-2-bromopropionate as initiator, Cu I , PMDETA and [CuII(PMDETA)Br2] complex and styrene. Stopping the polymerization at approximately 60% conversion in combination with added CuII complex produced polymer chains predominantly with Br end-groups. Both 1H NMR and MALDI−ToF (see Supporting Information, Figures S2 and S3, respectively) showed that most chains had Br end-groups. Polymer 2 had a number-average molecular weight (Mn) of 2690 and a narrow MWD with a polydispersity index (PDI) of 1.11 (see Table 1). The Br end-group of this polymer was then

20.00 g, 1.00 × 10−2 mol) was dissolved in pyridine (60 mL) with stirring, under Ar at 25 °C. The contents were cooled in an ice bath and tosyl chloride (19.06 g, 1.00 × 10−1 mol) was added portion-wise under positive argon flow. The reaction contents were allowed to slowly warm up to room temperature after addition and stirred overnight at room temperature. The contents turned an orange/yellow color. After stirring, the reaction mixture was diluted with cold water and extracted with dichloromethane (×3). The organic phases were washed with cold 6 M HCl (×2), dried with Na2SO4 and filtered. The solvent was reduced in volume under vacuum and the polymer solution precipitated into 10× volume of diethyl ether. The white, waxy solid was recovered by vacuum filtration and dried under vacuum. Synthesis of PEG47−N3 (9). NaN3 (3.25 g, 5.00 × 10−2 mol) was added to a stirring solution of PEG47-OTS (Mn ∼ 2000, 10.0 g, 5.00 ×

Table 1. Absolute MWD Data for Starting Polymers absolute Mw (triple detection) polymer

Mn

PDI

PSTY24−Br (2) PSTY24−N3 (3) c-PSTY28−Br (4) c-PSTY28−N3 (5) PtBA25−Br (6) PtBA25−N3 (7) PEG47−Br (8) PEG47−N3 (9)

2690 2670 3200 3170 3400 3440 2240 2180

1.11 1.11 1.06 1.09 1.11 1.12 1.06 1.07

reacted with NaN3 to give PSTY24−N3 (4) in high yields (see Supporting Information for characterization). Poly(tert-butyl acrylate) was also synthesized by ATRP to produce PtBA25−Br (6), in which the end-group was consequently converted to an azide to form PtBA25−N3 (7), and the Mn’s and PDIs given in Table 1 were based on absolute SEC molecular weight determination. Narrow MWD PEG polymers (PDIs < 1.07, Table 1), PEG47−Br (8) and PEG47−N3 (9) were synthesized from the starting PEG47−OH. Synthesis of cyclic PSTY followed a known literature procedure (see Scheme 2).17 First, the ATRP polymerization of styrene using an alkyne initiator gave a polymer with an Mn of 2950 and a low PDI of 1.08. There was no evidence for loss

10−3 mol) in DMF (50.0 mL) (milky white solution). The reaction mixture was stirred for 20 h at 25 °C. After stirring, the reaction mixture was poured into a brine solution and extracted with dichloromethane (×3). The organic layers were dried with Na2SO4 and filtered. The solvent was removed under reduced pressure. The residue was dissolved in dichloromethane and precipitated into 10x volume of diethyl ether. The white precipitate was collected via vacuum filtration and dried under vacuum. Linear PSTY calibrated SEC (Mn = 2800, PDI = 1.05) and triple detection SEC (Mn = 2200, PDI = 1.07). Synthesis of Miktoarm Stars (10−17). The General Procedure Used for the Synthesis of the Miktoarm Stars is Outlined Below. Trifunctional core (1) (1 equiv), polymer−Br (PSTY24−Br (2) or PtBA25−Br (6) or PEG47−Br (8) or c-PSTY28−Br (4)) (2.6 equiv), 5960

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Scheme 2. Synthetic Route for the Formation of Functional Cyclic c-PSTY (4, 5)a

(i) ATRP of STY at 80 °C in bulk, (ii) NaN3 in DMF for 20 h at 25 °C, (iii) CuBr, PMDETA in toluene at a polymer feed rate of 2.50 mL min−1 over 8 min, then stirred for 3 h at 25 °C, (iv) TEA in THF at RT for 56 h, (v) NaN3 in DMF for 20 h at 25 °C. a

of alkyne functionality (that could occur due to alkyne/alkyne coupling) during or after the polymerization, ensured by the thorough degassing of the reaction mixture with argon. The conversion of the Br end group to an azide was near quantitative (see Supporting Information). Cyclization of the difunctional PSTY was carried out at 25 °C by feeding a degassed mixture of polymer into a degassed solution of CuI and PMDETA over 8 min, and left to react for a further 3 h. The MWDs based on RI (i.e., PSTY calibration curve) before (curve a, Figure 1A) and after (curve b, Figure 1A) cyclization showed that the molecular weight at the peak maximum decreased. The change in hydrodynamic volume upon cyclic formation was 0.75, which is similar to that found in the literature.45 In curve a, there was a small amount of multiblock polymer. In order to determine the amount of monocyclic formed, we used a log-normal distribution (LND) model based on a Gaussian function to fit the experimental MWD. One can simulate the molecular weight distributions with a log-normal distribution46 using the following equations: fi =

2 2 1 e−(μ − log Mi) /2σ σ 2π

wi = fi Mi

⎛ M nM w μ = log⎜⎜ ⎝ PDI

(1) (2)

⎞ ⎟⎟ ⎠

(3)

⎛ log(PDI) ⎞1/2 σ=⎜ ⎟ ⎝ 2.303 ⎠

(4)

Figure 1. Molecular weight distributions (MWDs) for linear and cyclic PSTY (A) before and after cyclization of PSTY (a) linear alk(OH)− PSTY−N3, (b) after cyclization to form c-PSTY−OH, and (c) LND simulation of c-PSTY−OH with hydrodynamic volume change of 0.75. (B) MWDs of (a) SEC RI trace of crude c-PSTY−OH and (b) SEC RI trace of purified c-PSTY−OH by preparative SEC.

where eq 1 is given by the Gaussian distribution function, w(M) is the weight distribution of the SEC trace, Mn is the number-average molecular weight, Mw is the weight-average molecular weight, and the polydispersity PDI = Mw/Mn. The experimental MWD (w(M)) after cyclization can be simulated using the LND method using the experimental Mn and PDI values and the hydrodynamic change of 0.75 (see

curve c in Figure 1A). This provides a sensitive method to analyze the amount of monocyclic structure, and has been used effectively to analyze dendritic structures made from polymer building blocks42 and cyclic polymers.30,47 It can be seen in Figure 1 that the simulated MWD overlaps nearly perfectly with 5961

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Figure 2. Molecular weight distributions (MWDs) for starting polymers and linear miktoarms. (A) SEC RI MWDs of polymer precursors and product 10 (a) PSTY24−Br (2), (b) PtBA25-N3 (7), (c) crude miktoarm (10), and (d) purified AB2 miktoarm (10) by preparative SEC. (B) SEC RI MWDs of polymer precursors and product 11 (a) PEG47-N3 (9), (b) PSTY24−Br (2), (c) crude miktoarm (11), and (d) purified AB2 miktoarm (11) by preparative SEC. (C) SEC RI MWDs of polymer precursors and product 12 (a) PSTY24−N3 (3), (b) PtBA25-Br (6), (c) crude miktoarm (12), and (d) purified AB2 miktoarm (12) by preparative SEC. (D) SEC RI MWDs of polymer precursors and product 13 (a) PEG47-Br (8), (b) PSTY24−N3 (3), (c) crude miktoarm (13), and (d) purified AB2 miktoarm (13) by preparative SEC.

Table 2. Molecular Weight Data for Synthesis of AB2 Miktoarm Stars 10−17 after purification by prep SEC before purification by prep SEC

RI (PSTY calibration)

absolute Mw (triple detection)

product

% max. puritya

% purity (SEC)b

% coupling efficiencyc

Mn

PDI

Mn

PDI

ΔHDVd

10 11 12 13 14 15 16 17

81 80 76 77 82 82 81 81

81 81 81 72 78 81 79 81

>99 >99 >99 94 95 >99 97 >99

8760 8850 8740 9540 7590 7890 9070 8830

1.04 1.04 1.04 1.03 1.04 1.04 1.04 1.03

10 010 8410 10 120 7950 10 690 8940 10 900 8220

1.02 1.03 1.03 1.02 1.03 1.04 1.04 1.02

0.88 1.05 0.87 1.20 0.72 0.88 0.83 1.07

a

Calculated theoretically based upon the equivalents of polymer used compared to the equivalents of core used. bDetermined by dividing the area of the product SEC peak at 262 nm absorbance over the total area of all the peaks from the SEC at 262 nm (using a weight distribution). cDetermined by dividing purity (SEC) by max purity. dΔHDV = change in hydrodynamic volume (calculated from RI (PSTY calibration) and absolute MW TD).

polymer was obtained by fractionation through the preparative SEC column (Figure 1B, Mn = 3190 and PDI = 1.07), removing low and high molecular weight impurities. Conversion of cPSTY28−OH to c-PSTY28−Br (4) with the addition of bromopropionyl bromide was in near quantitative yields, and

the MWD after cyclization, allowing us to calculate a 93% purity for the monocyclic polymer. The peak observed at a lower molecular weight to the cyclic was most probably due to residual ligand and copper. For this functional polymer to be used in the synthesis of AB2 miktoarm stars, the cyclic polymer was further purified using preparative SEC. Pure monocyclic 5962

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Figure 3. Molecular weight distributions (MWDs) for starting polymers and linear and cyclic miktoarms. (A) SEC RI MWDs of polymer precursors and product 14 (a) c-PSTY28−Br (4), (b) PtBA25-N3 (7), (c) crude miktoarm (14), and (d) purified AB2 miktoarm (14) by preparative SEC. (B) SEC RI MWDs of polymer precursors and product 15 (a) PEG47-N3 (9), (b) c-PSTY28−Br (4), (c) crude miktoarm (15), and (d) purified AB2 miktoarm (15) by preparative SEC. (C) SEC RI MWDs of polymer precursors and product 16 (a) c-PSTY28−N3 (5), (b) PtBA25-Br (6), (c) crude miktoarm (16), and (d) purified AB2 miktoarm (16) by preparative SEC. (D) SEC RI MWDs of polymer precursors and product 17 (a) PEG47-Br (8), (b) c-PSTY28−N3 (5), (c) crude miktoarm (17), and (d) purified AB2 miktoarm (17) by preparative SEC.

so to conversion of 4 to c-PSTY28−N3 (5) with NaN3. The MWD data for these two polymers is given in Table 1. Synthesis of 3-Arm AB2 Stars. The trifunctional core (1) containing two nitroxide and one alkyne moieties was synthesized using our previously reported procedure.42 This core provided a simple route for the synthesis of AB2 miktoarm stars in a one-pot reaction catalyzed by copper. Product 10 (in Scheme 1) was made by coupling two PSTY24−Br (2.6 equiv, see SEC curve a in Figure 2A) and one PtBA28−N3 (1.3 equiv., see SEC curve b in Figure 2A) to 1 (1 equiv) catalyzed using CuIBr (3 equiv) in a 50/50 v/v mixture of toluene and DMSO at 25 °C. The copper activity was chosen so that both the CuAAC and NRC reactions occurred at similar rates (a parallel process). This reaction condition produced dendritic structures rapidly and with high efficiency.42 After 0.5 h, product 10 formed with a purity of 81% and a coupling efficiency (=purity/ max. purity × 100%) of greater than 99% (Table 2). The RI SEC (i.e., PSTY calibration) of 10 shown in curve c (Figure 2A) demonstrated that a small amount of starting polymer remained with the majority of the polymer formed as the 3-arm product. Purification of the crude 3-arm star to remove starting polymer by preparative SEC was shown in curve d (Figure 2A), and from Table 2 the hydrodynamic volume change from RI vs absolute SEC detection was 0.88. It should be noted that MALDI−ToF could not be completed for all structures linked

through alkoxyamines as these degrade under MALDI conditions as described in our previous work.36 We used the same procedure to form products 11 to 13, all of which formed with very high coupling efficiencies and little evidence of 2-arm formation (see Table 2 and Figure 2B-D). The change in hydrodynamic volume (ΔHDV) varied dependent upon the type of linear polymer building blocks. When the arms consisted of either PSTY or PtBA, the change in hydrodynamic volume due to the more compact 3-arm star formation was close to 0.87. However, when only one PEG chain was coupled, the ΔHDV increased to 1.05. Having two PEG chains on core 1 further increased the ΔHDV to 1.20. PEG chains can extend their hydrodynamic coil conformation due to hydrogen bond bridges between adjacent oxygen atoms.48,49 The next set of 3-arm stars consisted of cyclic polystyrene chains attached to either linear PtBA or PEG. Products 14 to 17 were analogues of 11 to 13, in which linear PSTY was substituted for its cyclic analogue but with 28 styrene units. The coupling efficiencies were very high and greater than 95% (Table 2), and in all cases, the SEC chromatograms showed only remaining starting polymer and 3-arm stars with no evidence of 2-arm or other polymer species (Figure 3). We could further purify these products by preparative SEC (Figure 3, curve d). Preparative SEC allowed isolation of pure 3-arm polymer from both starting polymers and residual 2-arm 5963

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Figure 4. SEC RI molecular weight distributions (MWDs) of crude linear miktoarms and LND simulations. (A) MWDs of (a) crude AB2 miktoarm (10), (b) LND simulation of (10). (B) MWDs of (a) crude AB2 miktoarm (11), (b) LND simulation of (11). (C) MWDs of (a) crude AB2 miktoarm (12), (b) LND simulation of (12). (D) MWDs of (a) crude AB2 miktoarm (13) and (b) LND simulation of 13.

species. The change in hydrodynamic volume by RI to absolute detection was lower than when coupling with the linear PSTY (Table 2). This was due to the initial change in HDV due to monocyclic PSTY formation and the additional decrease in HDV due to 3-arm formation. Product 17 showed a slight increase in ΔHDV due to the two extended PEG chains. Supporting Information provides the characterization of all 3arm polymers. The recovered yield of miktoarms 10−17 was approximately 40%. The decrease in recovered yield can be attributed to some loss of polymer during the preparative-SEC process and also the subsequent precipitation of the nonhomopolymer miktoarms into solvent systems which can allow for some partial solubility. The additional advantage of making these structures was that PtBA can be easily converted to poly(acrylic acid) (data not shown), providing a simple method to amphiphilic stars. We have used characterization techniques, such as NMR and MALDI−ToF spectrocsopy, to analyze the loss and formation of end-groups of 3-arm stars. Although this provides some evidence for 3-arm formation, it is by no means conclusive, especially for polymers where coupling of polymer building blocks leads to much higher molecular weights. At such high molecular weights, the error from the NMR analysis and other techniques is not insignificant. A better and more accurate methodology is to use the above characterization technique and also use the LND method to analyze the experimental MWD, as previously described for analyzing cyclic polymers (Figure

1). The MWD simulated was based on the weight distribution (i.e., w(M)) as the total weight of starting polymers was equal to the total combined weight of product 3-arm star and other polymer species (e.g., starting and 2-arm) after coupling. In the LND simulations, we used a theoretical Mn of the 3-arm star by adding the Mn’s of the starting polymers (Table 1), and using the ΔHDV in Table 2. The PDI values for the coupling of three polymer chains was determined using the following equation:50 PDI3 ‐ arm = 1 + ((PDIA − 1)wA 2 + (PDIB − 1)wB 2 + (PDIC − 1)wC 2)

(5)

where wA, wB, and wC are the weight fractions of A, B, and C in the 3-arm star (i.e., wA = 1 − wB − wC). The PDI for the 3-arm star using eq 5 was calculated from the individual PDI values in Table 1. This allowed us to determine the formation and yield of 3arm stars and further determine the amount if any of 2-arm species. We did not fit the LND distributions to the 3-arm stars after purification by preparative SEC, since one can obtain a slight shift in the molecular weight distribution after fractionation. Therefore, the values (Mn and PDIs) used to determine the LND molecular weight distribution were all obtained from experiment, and importantly no adjustable parameters were used in the fit. The LND MWDs given in Figures 4 and 5 showed excellent fits with the experimental RISEC MWDs. In nearly all cases for the linear building blocks 5964

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Figure 5. SEC RI molecular weight distributions (MWDs) of crude linear and cyclic miktoarms and LND simulations. (A) MWDs of (a) crude AB2 miktoarm (14) and (b) LND simulation of (14). (B) MWDs of (a) crude AB2 miktoarm (15) and (b) LND simulation of (15). (C) MWDs of (a) crude AB2 miktoarm (16) and (b) LND simulation of (16). (D) MWDs of (a) crude AB2 miktoarm (17) and (b) LND simulation of (17).

change in hydrodynamic volumes provided an additional and sensitive characterization method to the coupling efficiency for 3-arm formation. This method allows one to determine the type and amount of starting, two-arm or other high molecular weight species after coupling. There were no adjustable parameters used in the LND simulations, making this a powerful method to characterize not only cyclic polymers but more complex architectures like 3-arm stars. The LND simulations gave excellent agreement with the experimental MWDs, allowing us to determine the coupling efficiency of greater than 99% in most cases. When the cyclic PSTY was used as a building block the coupling efficiency was still very high, but due to the presumably slower CuAAC reaction, higher molecular weight species thought to be formed through alkyne−alkyne coupling were observed.

(Figure 4), there was little or no 2-arm formation. In the case where the c-PSTY was used, the LND MWD showed that there was a very small amount of 2-arm (less than 2%) and some higher molecular weight polymer most probably due to alkyne−alkyne coupling. This was only evident when c-PSTY was used, suggesting that steric hindrance in using c-PSTY could lead to a slower CuAAC reaction, resulting in alkyne− alkyne coupling becoming kinetically competitive. The LND method used above is a powerful tool for determining the purity of complex polymer architectures formed through coupling building blocks.

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CONCLUSION In this work, we have demonstrated the rapid (in 30 min) and highly efficient (at close to 99%) one-pot synthesis of mikto 3arm AB2 star at 25 °C. Well-defined linear and cyclic building blocks consisting of linear-PSTY, c-PSTY, PtBA, and PEG were used in a variety of combinations to make such AB2 stars. Two “click”-type reactions (i.e., CuAAC and NRC) were used to couple the polymers onto a core functionalized with two free nitroxides and one alkyne group (1). The star incorporated with PEG showed an increased in the SEC change in hydrodynamic volume due to the more linear conformation of the PEG in solvent. All other stars showed a decreased in the hydrodynamic volume. The LND method to simulate the molecular weight distribution using experimental Mn, PDI and

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis and characterizations of all the TEMPO derivatives, experimental details of end group modification reactions, all the 1 H NMR spectra, ATR-FTIR spectra, MALDI−ToF, ESI−MS spectra. This material is available free of charge via the Internet at http://pubs.acs.org. 5965

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.J.M acknowledges financial support from the ARC Discovery grant (DP0987315), Z.J. acknowledges the UQ Postdoctoral Research Fellowship.

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